1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and an image correction estimating method which excite nuclear spin of an object magnetically with a RF signal having the Larmor frequency and reconstruct an image based on a magnetic resonance signal generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and an image correction estimating method, which estimate whether correction for image distortion due to nonuniformity on a magnetic field was performed appropriately.
2. Description of the Related Art
Conventionally, a MRI (Magnetic Resonance Imaging) apparatus is used for a monitoring apparatus in medical field.
The MRI apparatus is an apparatus which magnetically resonates nuclear spins in an object by transmitting a RF (radio frequency) signal from a RF coil with forming gradient magnetic fields by gradient coils in X-axis, Y-axis and Z-axis directions on an imaging area of the object set in a cylindrical magnet generating a static magnetic field and reconstructs an image of the object using a nuclear magnetic resonance (NMR) signal generated due to the excitation.
The MRI apparatus as described above has a problem that uniformity of intensities of a static magnetic field is partly disturbed by the performance limitation of a magnet for static magnetic field forming a static magnetic field and the variation in magnetic susceptibility when an object enters a magnetic field. It is well-known that a reconstructed image is distorted when the uniformity of intensities of a static magnetic field is disturbed.
The serious influence from nonuniformity of intensities of a static magnetic field is affected in the case of imaging on EPI (echo planar imaging) method. The EPI method is a super-fast scan method for obtaining all data for reconstruction of image by one-time excitation pulse, and is also called single-shot EPI method. The imaging using the EPI method is performed so as to prevent the influences from the body movement and the pulsating of an object in the case of drawing the cerebral infarction in the examination of the head of the object. However, a diffusion weighted image obtained by using the EPI method is greatly influenced from the nonuniformity of intensities of a static magnetic field. When the intensities of a static magnetic field are not sufficiently uniform, the reconstructed image is distorted.
Then, in order to solve the problem caused by the nonuniformity of intensities of a static magnetic field, according to a conventional art, the distribution of intensities of a static magnetic field is estimated, and an image position and an image value of the reconstructed image are corrected based on the above-estimated distribution of intensities of the static magnetic field (for example, refer to a document, “Geometric distortion in MRI for stereotactic neurosurgery” on pp. 749 to 765 of Magnetic Resonance Imaging, Volume 12, Number 5, 1994 written by J. MICHIELS ET AL).
There are three factors for disturbing uniformity of intensities of a static magnetic field and distorting reconstructed image. That is, a first factor is an error of a gradient magnetic field, a second factor is an error of a static magnetic field due to only a magnet for static magnetic field, and a third factor is an error of the static magnetic field due to an object entering the static magnetic field. Therefore, the error of the static magnetic field is defined by the total of the error of the gradient magnetic field, the error of the static magnetic field due to only the magnet for static magnetic field, and the error of the static magnetic field due to the object which enters the static magnetic field.
Further, the amount of distortion of the image data is obtained every position in an image space from the error of the static magnetic field, an image value is corrected every position in the image space based on the obtained amount of distortion of the image data. That is, in the image space, the three-dimensional positions each having an image value are first shifted to the positions without the distortion. However, the shifted positions are not necessarily on the original grid points, and image values on the grid points are therefore obtained by the interpolation using a plurality of image values at the shifted positions.
In the conventional correction of the image distortion due to the nonuniform intensities of a static magnetic field, importantly in precision, the distribution of distortion amounts of the static magnetic field is estimated in accordance with the error factors of the static magnetic field. In particular, since the error of the static magnetic field which is caused by the entering of the object in the static magnetic field varies depending on the movement of the object, estimated values of the error distribution of the static magnetic field can include errors.
Then, in the image correction based on the error distribution of the static magnetic field without precisely estimating the error distribution of the static magnetic field, there is a danger that the image distortion is not appropriately improved. Therefore, such the development of a technique is expected whether or not the error distribution of the static magnetic field is precisely estimated, alternatively whether or not the image distortion due to the nonuniformity of a static magnetic field is appropriately corrected.